Synthesis of a potential new class of radiotracers, metal cryptate complexes of trivalent metal ion radioisotopes, has been effected. This project will explore and develop in vivo and in vitro applications of these new materials. In particular, it now seems clear that positron axial tomography (PAT) techniques will soon provide better images of tissues, blood vessels and organs in vivo than now available from gamma camera pictures. Several isotopes which form chemically stable trivalent ions could be most useful as positron sources. Recent studies indicate that 68Ga(III) ion is likely to be the most important. Its chemical and physical properties endow it with the potential for supplanting 99mTc as the most medically useful radioisotope. The gallium cryptates and the metal cryptate complexes of several lanthanide and actinide ions as well as those of In(III) were first explored during this past year. Experiments completed to date include the development of safe techniques for encryptating radioisotopes, the synthesis of Ce(III) cryptates, the first in vivo injections of that trivalent cryptate, and the acquisition of gamma-camera images to explore in vivo distribution of metal cryptates.